Device roaming in hybrid Wi-Fi/wireline and multi-AP networks

ABSTRACT

A method for communication includes communicating between a first transceiver and a wireless communication terminal on a first channel, while identifying the first transceiver to the terminal with a transceiver identity. A roaming condition, which determines whether the terminal is to roam to a second transceiver operating on a second channel, is evaluated. Upon meeting the roaming condition, the terminal is instructed to switch to the second channel, and the terminal is subsequently communicated with using the second transceiver on the second channel while identifying the second transceiver to the terminal with the same transceiver identity as the first transceiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 61/494,990 , filed Jun. 9, 2011 , whose disclosure isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to wireless communication, andparticularly to methods and systems for device roaming in wirelesscommunication networks.

BACKGROUND OF THE INVENTION

Many wireless communication networks support roaming processes forallowing mobile communication devices to transition from one basestation to another. For example, roaming in IEEE 802.11 Wireless LocalArea Networks (WLAN) is specified in IEEE Standard 802.11r-2008,entitled “IEEE Standard for Information Technology—Telecommunicationsand Information Exchange between Systems—Local and Metropolitan AreaNetworks-Specific Requirements Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications Amendment 2: FastBasic Service Set (BSS),” 2008, which is incorporated herein byreference.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesa method for communication. The method includes communicating between afirst transceiver and a wireless communication terminal on a firstchannel, while identifying the first transceiver to the terminal with atransceiver identity. A roaming condition, which determines whether theterminal is to roam to a second transceiver operating on a secondchannel, is evaluated. Upon meeting the roaming condition, the terminalis instructed to switch to the second channel, and the terminal issubsequently communicated with using the second transceiver on thesecond channel while identifying the second transceiver to the terminalwith the same transceiver identity as the first transceiver.

In some embodiments, the first and second transceivers include AccessPoints (APs) of a Wireless Local Area Network (WLAN), and thetransceiver identity includes at least one of a Service Set Identifier(SSID) and a Basic Service Set Identifier (BSSID). In an embodiment,instructing the terminal includes sending to the terminal a channelchange command that instructs the terminal to change the channel but toremain associated with the same transceiver.

In a disclosed embodiment, communicating with the terminal using thesecond transceiver includes establishing communication between thesecond transceiver and the terminal using same communication parametersused for communication between the first transceiver and the terminal.In another embodiment, communicating with the terminal using the secondtransceiver includes establishing communication between the secondtransceiver and the terminal without negotiating communicationparameters between the second transceiver and the terminal. In yetanother embodiment, communicating with the terminal using the secondtransceiver includes transferring to the second transceiver one or morecommunication parameters used for communication between the firsttransceiver and the terminal, and causing the second transceiver tocommunicate with the terminal using the communication parameters.

In some embodiments, after switching to the second channel, the methodincludes determining that the second transceiver is unsuitable forcommunicating with the terminal, and taking an alternative action.Taking the alternative action may include instructing the terminal toswitch back to the first transceiver, instructing the terminal to switchto a third transceiver, initiating communication with the terminal overan alternate communication protocol, or denying communication with theterminal. In an embodiment, evaluating the roaming condition includesestimating a quality metric for the first channel, and deciding whetherto roam to the second channel based on the estimated quality metric.

In some embodiments, evaluating the roaming condition includes balancinga traffic load between the first and second transceivers. In an exampleembodiment, evaluating the roaming condition includes assessing atraffic need of the terminal, and deciding whether to roam to the secondchannel based on the assessed traffic need. Additionally oralternatively, evaluating the roaming condition may include assessing atraffic capability of the first or second transceiver, and decidingwhether to roam to the second channel based on the assessed trafficcapability.

In an embodiment, before evaluating the roaming condition, the methodincludes carrying out a channel selection process that assigns multiplechannels to multiple respective transceivers, including assigning thefirst and second channels to the first and second transceivers,respectively. Carrying out the channel selection process may includeassigning a given channel to at least two of the multiple transceivers.The method may include coordinating access to the given channel amongthe two or more transceivers assigned to the given channel.

In another embodiment, carrying out the channel selection processincludes disabling one or more of the multiple transceivers in order toassign each transceiver that is not disabled a different respectivechannel. In yet another embodiment, the method includes distributingmultiple terminals, including the wireless communication terminal, amongthe multiple transceivers by evaluating the roaming condition andinstructing the wireless communication terminal to switch.

There is additionally provided, in accordance with an embodiment of thepresent invention, a communication system including at least first andsecond transceivers and at least one processor. The first and secondtransceivers are configured to communicate with wireless communicationterminals on respective first and second channels, while bothidentifying to the terminals with a transceiver identity. The processoris configured to evaluate a roaming condition that determines whether awireless communication terminal is to roam from the first transceiver tothe second transceiver, and, upon meeting the roaming condition, toinstruct the terminal to switch from the first channel to the secondchannel, and to cause the second transceiver to subsequently communicatewith the terminal while identifying to the terminal with the sametransceiver identity as the first transceiver.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a wirelesscommunication system, in accordance with an embodiment of the presentinvention;

FIG. 2 is a flow chart that schematically illustrates a method forroaming in a wireless communication system, in accordance with anembodiment of the present invention; and

FIG. 3 is a flow chart that schematically illustrates a method forinitial channel assignment in a wireless communication system, inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention that are described herein provideimproved methods and systems for roaming in wireless networks, such asWireless Local Area Networks (WLANs). In some embodiments, a wirelesscommunication system comprises multiple transceivers that cover acertain geographical area and provide communication services to wirelesscommunication terminals. Amongst other functions, the system supports ahighly-efficient roaming process that enables terminals to transitionfrom one transceiver to another rapidly and with little or no loss ofdata.

In the embodiments described herein, the transceivers comprise WLANAccess Points (APs) and the terminals comprise WLAN Stations (STAB). Thedisclosed techniques, however, are applicable to various other types ofwireless networks.

In some embodiments, multiple transceivers in the system (in the presentexample APs) identify to the communication terminals (in the presentexample STAB) with the same transceiver identity or identifier (e.g., asingle Service Set Identifier (SSID) or a single Basic SSID (BSSID) in aWLAN implementation). Thus, from the point of view of the STAB, thedifferent APs appear logically as a single AP.

In a typical embodiment, a STA initially communicates with an AP denotedAP A on a certain channel. A processor in the system (e.g., in one ofthe APs or in any other system element) evaluates a roaming condition soas to determine that the STA is to roam to a different AP denoted AP B.Upon meeting the roaming condition, AP A instructs the STA to switch tothe channel of AP B, and communication is subsequently resumed with theSTA via AP B.

Since both AP A and AP B identify to the STA using the same AP identity(e.g., SSID or BSSID), the STA is typically unaware that roaming tookplace. From the point of view of the STA, the entire roaming processappears as a channel switch.

In some embodiments, AP A and AP B communicate with one another overwireless or wireline media, and are thus able to coordinate the roamingprocess between them. For example, AP A may notify AP B of the imminentroaming, and provide AP B with attributes of the STA and/or the link tothe STA. This coordination enables the APs to perform seamless or nearlyseamless roaming, without a need for cooperation or awareness on thepart of the STA.

The processor may initiate the roaming process based on various kinds ofroaming conditions. The roaming condition may depend, for example, onthe link quality between the APs and the STA, and/or on traffic needs orcapabilities of the STAB and/or APs. As another example, the processormay assign STAB to APs based on some policy or criterion, and instructthe STAB to roam to their designated APs using the disclosed roamingprocess.

In summary, the roaming processes described herein enables fast roamingwith little or no loss of data, and are typically transparent to theSTAB. The roaming process can be used with conventional WLAN STABwithout a need for hardware or software modification. The disclosedtechniques can be used in various wireless network applications, such asin hybrid wireless/wireline home network configurations.

System Description

FIG. 1 is a block diagram that schematically illustrates a wirelesscommunication system 20, in accordance with an embodiment of the presentinvention. System 20 comprises multiple WLAN APs 24 that communicatewith WLAN STAB 28 in a certain geographical area.

In the present example, APs 24 and STAB 28 operate in accordance withthe IEEE 802.11 WLAN standards. STAB may comprise any suitableWLAN-capable devices. In alternative embodiments, however, the disclosedtechniques can be used in various other wireless systems, which anysuitable type of transceivers and communication terminals.

In the configuration of FIG. 1, although not necessarily, APs 24 areinterconnected by a network 32. Network 32 may comprise, for example, awireline network such as an IEEE 802.3 Local Area Network (LAN), apowerline communication network, a Multimedia over Coax Alliance (MoCA)network, or any other suitable network type. Alternatively, network 32may comprise a suitable wireless network. In one example embodiment, theAPs comprise dual-band devices, communicating with the STAB over a 5 GHzWLAN and with one another over a 2.4 GHz WLAN.

In the present example, system 20 comprises a hybrid wireline/wirelessnetwork for the home, i.e., APs 24 cover different rooms or areas in agiven home. Communication traffic, e.g., packets or frames, enters andexits the home via network 32, and is relayed to STAB using APs 24.Alternatively, system 20 may comprise any other suitable system typethat is deployed in any other suitable indoor or outdoor environment.

System 20 may comprise any suitable number of APs 24. The presentexample shows three APs denoted AP A, AP B and AP C. Each AP 24comprises a respective transceiver 36 for communicating with STAB 28 viaan antenna array 40, and a respective processor 44 that manages theoperation of the AP.

As will be explained in detail below, system 20 supports ahighly-efficient roaming process that enables STAB 28 to roam from oneAP to another. The roaming process may be carried out by one or more ofprocessors 44 of APs 24, by a separate processor (not shown) that isexternal to the APs, and/or by any other suitable processor. The roamingprocess may be partitioned between such processors in any suitablemanner. For the sake of clarity, the description that follows sometimesrefers to “a processor” that carries out the roaming process. Thisreference means to cover any suitable implementation using one or moreprocessors, either internally to the APs or otherwise.

The configuration of system 20 shown in FIG. 1 is an exampleconfiguration, which is chosen purely for the sake of conceptualclarity. In alternative embodiments, any other suitable systemconfiguration can be used. Some elements of system 20 may be implementedin hardware, e.g., in one or more Application-Specific IntegratedCircuits (ASICs) or Field-Programmable Gate Arrays (FPGAs). Additionallyor alternatively, some elements of system 20, such as processors 44, canbe implemented using software, or using a combination of hardware andsoftware elements.

Some of the functions of system 20, such as some or all of the functionsof processors 44, may be carried out using one or more general-purposeprocessors, which are programmed in software to carry out the functionsdescribed herein. The software may be downloaded to the processors inelectronic form, over a network, for example, or it may, alternativelyor additionally, be provided and/or stored on non-transitory tangiblemedia, such as magnetic, optical, or electronic memory.

Device Roaming Schemes

In some embodiments, system 20 supports a roaming process fortransitioning STAB 28 from one AP 24 to another. Each AP 24 typicallyoperates on a different channel, e.g., on a different frequency. All theAPs, however, identify to the STAs using the same AP identifier (e.g.,the same BSSID or SSID).

From the point of view of the roaming STA, the disclosed roaming processappears as a switch of channel. The roaming STA is typically unaware ofthe fact that roaming takes place. In other words, the roaming STA isunaware of whether the serving AP following the channel switch is thesame as or different from the serving AP before the channel switch. Thisprocess can therefore be used with conventional or legacy STAs, withouta need for modification or upgrade.

FIG. 2 is a flow chart that schematically illustrates a method forroaming in system 20, in accordance with an embodiment of the presentinvention. As explained above, the description that follows refers to “aprocessor” that carries out the roaming process, but in practice theprocessor functions may be implemented in one or more processors 44 ofthe APs, or in any other suitable processor.

The method begins with APs 24 of system 20 communicating with STAs 28,at a communication step 50. Typically, all the APs identify to the STAsusing the same AP identifier (BSSID or SSID), so as to appear to theSTAs logically as a single AP. At any given point in time, each STA 28is associated with a particular AP referred to as the serving AP of theSTA.

Consider a given STA that is served by AP A in FIG. 1. The processor(typically processor 44 of AP A) tracks the link quality of thecommunication channel between the AP and the STA, at a quality trackingstep 54. This step is typically performed by each AP for each of itsassociated STAs. The processor may assess the link quality directly,e.g., by performing measurements on the reverse channel from the STA tothe AP, or indirectly by receiving from the STA feedback relating to theforward channel from the AP to the STA. The processor may assess thelink quality using any suitable metric, which may relate to signalquality, to Quality of Service (QoS) or to any other property that isindicative of the link quality between the AP and the STA. Such metricsmay comprise, for example, Signal to Noise Ratio (SNR), Packet ErrorRate (PER), delay, jitter, and/or any other suitable metric orcombination of metrics.

The processor checks whether the assessed link quality is sufficientlyhigh, at a quality checking step 58. For example, the processor maycompare the assessed link quality to a predefined threshold. If the linkquality is sufficient for allowing the STA to remain associated with thesame AP, the method loops back to step 50 above.

Otherwise, the processor initiates a roaming process that willtransition the STA from AP A to another AP, in the present example to APB. Initially, the processor pauses the communication between AP A andthe STA, at a pausing step 62, and notes the last frame that was sent tothe STA. The processor notifies AP B that the STA is about to roam toit, at a notification step 66.

In some embodiments, the processor also sends to AP B one or moreattributes of the STA or of the communication with the STA, so thatcommunication between the STA and AP B can be resumed rapidly andwithout negotiation. AP A may send the notification and/or attributes toAP B over any suitable communication means between them, e.g., an IEEE802.3 link, MoCA link, powerline link or another IEEE 802.11 WLAN linkin a different band, when the APs comprise dual-band devices.

The transferred attributes may comprise, for example, link budgetmetrics for the STA, traffic description for the STA (e.g., priority oraverage traffic bandwidth), STA capabilities (e.g., MAC and PHYcapabilities), status and content of STA queues, block ACK sessiondetails for the STA, security keys used in communication with the STA,indication of the last successfully-delivered frame, and/or any othersuitable attribute.

The processor then causes AP A to instruct the STA to switch to thechannel of AP B, at a channel switching step 70. In an exampleembodiment, the instruction comprises a channel change command asspecified in the IEEE 802.11h standard. This command is typically nodifferent from a command to change a channel vis-à-vis the same AP, andit does not indicate to the STA that roaming is to take place. After theSTA has switched to the channel of AP B, the processor causes AP B toevaluate the link quality between AP B and the STA, at a qualityevaluation step 74.

AP B may evaluate any suitable link quality metric for this purpose,such as any of the metrics used at step above. In alternativeembodiments, AP B may measure the Received Signal Strength Indication(RSSI) on the reverse channel, or evaluate the forward channel byperforming a sounding operation and receiving feedback from the STA.

Based on the evaluated channel, the processor decides whether the STA isto remain associated with AP B or not, at a roaming checking step 78.The decision in step 78 may be taken by AP B based on the information itreceived from AP A, e.g., at step 66. Alternatively, the decision can betaken by AP A based on information regarding the new channel provided byAP B. Further alternatively, the decision can be taken by any otherelement in the system, and conveyed back to AP B.

If the decision is to have the STA remain with AP B, the processorcauses AP B to resume communication with the STA, at a resumption step82. AP B typically resumes the communication using the attributesprovided by AP A at step 66 above. As such, the link can be establishedseamlessly without re-negotiation with the STA, with small latency andwith little or no loss of data. In some embodiments AP B notifies AP Aof the decision, and of the newly measured link quality metrics.

In some embodiments, AP B requests some of the STA attributes, forexample STA data queue content, after the decision of step 78 is taken.This feature can be useful, for example, when the data size in questionis large and it is preferable not to ask for it in advance, in case theSTA will not remain with AP B. The method then loops back to step 50above.

If, on the other hand, the decision in step 78 is that the STA is not toremain with AP B, the processor concludes that the roaming attempt hasfailed and takes alternative action, at a failure step 86. One possiblealternative action is to return the STA to communicate with AP A. Inthis embodiment, AP B typically reports the decision to AP A. AP B mayalso report the reason for the decision, e.g., poor link quality on thechannel of AP B. The STA is then instructed to switch back to thechannel of AP A, e.g., using a DFS channel change command. AP A can thenresume communication with the STA.

Another possible alternative action is to hand off the STA to yetanother AP (e.g., AP C). Yet another possibility is to deny service fromthe STA, e.g., when dynamic admission control is supported. When system20 comprises a hybrid network (e.g., in accordance with the emergingP.1950.1 standard), and/or when the STA is reachable using an alternatecommunication protocol (e.g., powerline communication), the alternativeaction may be to establish communication with the STA over the alternateprotocol and not via APs 24.

The method flow of FIG. 2 is an example flow, which is chosen purely forthe sake of conceptual clarity. In alternative embodiments, theprocessor may apply the disclosed roaming process using any othersuitable sequence of steps.

For example, the decision to transfer the STA from AP A to AP B based onlink quality metrics is only a specific example of a roaming conditionthat can be evaluated by the processor. In alternative embodiments, theprocessor may decide to transfer an STA from one AP to another byevaluating any other suitable roaming condition. The roaming conditionneed not necessarily depend on link quality and may depend on otherfactors, such as bandwidth needs and capabilities of the APs and/orSTAB.

In one example embodiment, the processor may evaluate the roamingcondition periodically for the various STAB, irrespective of their linkquality. This sort of solution may be used for dynamically reassigningSTAs to APs. Moreover, such a solution may prevent the link quality fromdeteriorating in the first place.

Initial Channel Selection

In some embodiments, the roaming condition is used for defining andapplying a dynamic assignment of STAs to APs. In other words, system 20may use the disclosed roaming process to perform initial assignment andsubsequent reassignment of STAs to APs. The coordination between APs inthis process may be performed over any suitable communication medium,e.g., over the same 5 GHz band used for communicating with the STA, overanother wireless link such as a 2.4 GHz link, or using a wirelinemedium.

In a typical initial channel selection process, after system 20 isswitched on each AP 24 in the system makes an initial selection of achannel to operate on. The channel selection process is typically,although not necessarily, performed by cooperation between the differentAPs.

Typically, the APs select their respective channels without priorknowledge of the identities, numbers or locations of STAs 28. In anexample embodiment, each AP scans the frequency spectrum, evaluates theinterference conditions, possibly identify the currently-existing STAs,and then decide (jointly or independently) on the channel to be selectedby each AP.

In some embodiments, each AP is assigned a different channel. In otherembodiments, there is insufficient spectrum for such an assignment, andthus two or more of the APs need to be assigned the same channel.

The latter embodiments may involve scheduling or other coordinationamong the APs that use the same channel, particularly since all the APsidentify using the same AP identifier (e.g., BSSID). In such a scenario,identical beacon frames sent by different APs may confuse the STAB.

In an example embodiment, the processor coordinates the transmissions ofthe APs on a given channel to occur in non-overlapping time slots, inorder to avoid collisions. In another embodiment, the processor disablesone or more of the APs in the system, thereby eliminating the need tooperate multiple APs on the same channel. In this embodiment, theprocessor may dynamically activate and deactivate APs according tochannel and traffic conditions to the different STAB.

In some embodiments, when multiple APs share the same channel and usethe same AP identifier, the MAC modules of the APs support a mechanismthat identifies which STAB on the channel is served by which AP. Forexample, each AP is to send acknowledgement (ACK) frames only to theSTAB it serves.

After making the initial channel selection, the STAB are free to choosewhich AP to associate with according to their own channel selectionscheme. After an STA has associated on a certain channel, the AP on thatchannel (or the processor in general) may instruct the STA to switch tothe channels of other APs in order to evaluate the link metrics on thosechannels.

Based on this information, the processor may assign the STA to anappropriate channel (and thus AP). Typically, it can be assumed thateach STA will choose the channel (and thus AP) that will provide it withthe best link metric. The processor may later change this channelselection for reasons of load balancing, since the STA is typicallyunaware of the overall system traffic needs upon initial scan andassociation.

FIG. 3 is a flow chart that schematically illustrates a method forinitial channel assignment in system 20, in accordance with anembodiment of the present invention. The method begins with each AP 24scanning the at least some of the frequency channels of system 20 andevaluating the link quality to at least some of the STAB, at a channelscanning step 90.

The processor defines an initial assignment of STAB to APs based on theevaluated link quality, at an assignment step 94. The assignment mayalso depend on traffic needs of the different STAB and/or the trafficcapabilities (e.g., available capacity) of one or more of the APs. Theprocessor instructs each STA to roam to its designated AP, in accordancewith the assignment, at a roaming instruction step 98. This step istypically implemented using the roaming process defined above. Themethod of FIG. 3 can be used for initial channel selection, as well asfor dynamic reassignment of STAB to APs to match varying conditions,varying locations and varying traffic needs, for example.

In one embodiment, the STAB initially associate with APs in accordancewith a certain default channel selection, typically regardless of thelocations of the STAB and APs. Then, the method of FIG. 3 is used toreassign the STAB to the APs so as to optimize link quality and/ortraffic needs. The assignment can take traffic needs into account ifthis information is known to the PAs, for example if TSPEC is availableper stream.

When implementing the channel scanning of step 90, each STA may beinstructed to switch to the channels of the various APs, and the APs arethen able to evaluate the link quality to the various STAB. When alllink quality metrics are collected, the STAB are re-distributed amongthe APs using the disclosed roaming process.

When implementing the roaming process described herein, a deadlock mayoccur if an STA is instructed to switch to the channel of a new AP, butthe link quality on the new channel is so poor that communication cannotbe established with the new AP. In one embodiment, the STA overcomesthis problem by performing a re-scan when it fails to establishcommunication with the new AP.

In an alternative embodiment, the new AP detects this situation and,after a certain time-out period from the time it was notified that theSTA is supposed to join, reports back to the old AP that the roamingoperation failed. The old AP can then temporarily switch to the channelof the new AP, resume communication with the STA and instruct it toswitch back to the channel of the old AP. The old AP then returns to itsown channel and resumes operation. Further alternatively, when the STAis aware of the roaming operation, it can choose to return to thechannel of the old AP after a time-out period.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and sub-combinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art. Documents incorporated by reference in the present patentapplication are to be considered an integral part of the applicationexcept that to the extent any terms are defined in these incorporateddocuments in a manner that conflicts with the definitions madeexplicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

The invention claimed is:
 1. A method for communication, comprising:configuring multiple different transceivers, including at least firstand second transceivers operating on respective first and secondchannels, with a single transceiver identifier, such that the multipletransceivers appear to wireless communication terminals as a singletransceiver; communicating between the first transceiver and a wirelesscommunication terminal on the first channel, while identifying the firsttransceiver to the terminal with the single transceiver identifier; andwhen the terminal is to roam, sending to the terminal a channel switchcommand to switch to the second channel, which appears to the terminalas a channel switch confined to the single transceiver but which causesthe terminal to start communicating with the second transceiver insteadof with the first transceiver, while identifying the second transceiverto the terminal with the single transceiver identifier.
 2. The methodaccording to claim 1, wherein the first and second transceivers compriseAccess Points (APs) of a Wireless Local Area Network (WLAN), and whereinthe single transceiver identifier comprises at least one of a ServiceSet Identifier (SSID) and a Basic Service Set Identifier (BSSID).
 3. Themethod according to claim 1, and comprising, subsequently to sending thechannel switch command, establishing communication between the secondtransceiver and the terminal using same communication parameters usedfor communication between the first transceiver and the terminal.
 4. Themethod according to claim 1, and comprising, subsequently to sending thechannel switch command, establishing communication between the secondtransceiver and the terminal without negotiating communicationparameters between the second transceiver and the terminal.
 5. Themethod according to claim 1, The method according to claim 1, andcomprising, subsequently to sending the channel switch command,transferring to the second transceiver one or more communicationparameters used for communication between the first transceiver and theterminal, and causing the second transceiver to communicate with theterminal using the communication parameters.
 6. The method according toclaim 1, and comprising, after switching to the second channel,determining that the second transceiver is unsuitable for communicatingwith the terminal, and taking an alternative action.
 7. The methodaccording to claim 6, wherein taking the alternative action comprisesone of: instructing the terminal to switch back to the firsttransceiver; instructing the terminal to switch to a third transceiver;initiating communication with the terminal over an alternatecommunication protocol; and denying communication with the terminal. 8.The method according to claim 1, and comprising estimating a qualitymetric for the first channel, and deciding whether to roam to the secondchannel based on the estimated quality metric.
 9. The method accordingto claim 1, wherein sending the channel switch command comprisesbalancing a traffic load between the first and second transceivers. 10.The method according to claim 1, and comprising assessing a traffic needof the terminal, and deciding whether to roam to the second channelbased on the assessed traffic need.
 11. The method according to claim 1,and comprising assessing a traffic capability of the first or secondtransceiver, and deciding whether to roam to the second channel based onthe assessed traffic capability.
 12. The method according to claim 1,and comprising, before deciding that the terminal is to roam, carryingout a channel selection process that assigns multiple channels to themultiple respective transceivers, including assigning the first andsecond channels to the first and second transceivers, respectively. 13.The method according to claim 12, wherein carrying out the channelselection process comprises assigning a given channel to at least two ofthe multiple transceivers.
 14. The method according to claim 13, andcomprising coordinating access to the given channel among the two ormore transceivers assigned to the given channel.
 15. The methodaccording to claim 12, wherein carrying out the channel selectionprocess comprises disabling one or more of the multiple transceivers inorder to assign each transceiver that is not disabled a differentrespective channel.
 16. The method according to claim 12, and comprisingdistributing multiple terminals, including the wireless communicationterminal, among the multiple transceivers by evaluating a roamingcondition and instructing the wireless communication terminal to switch.17. A communication system, comprising: multiple transceivers, includingat least first and second transceivers operating on respective first andsecond channels, which are configured with a single transceiveridentifier such that the multiple transceivers appear to wirelesscommunication terminals as a single transceiver; and at least oneprocessor, which is configured to send to a wireless communicationterminal a channel switch command to switch from the first channel tothe second channel, which appears to the terminal as a channel switchconfined to the single transceiver but which causes the terminal tostart communicating with the second transceiver instead of with thefirst transceiver, while identifying to the terminal with the sametransceiver identity as the first transceiver.
 18. The system accordingto claim 17, wherein the first and second transceivers comprise AccessPoints (APs) of a Wireless Local Area Network (WLAN), and wherein thesingle transceiver identifier comprises at least one of a Service SetIdentifier (SSID) and a Basic Service Set Identifier (BSSID).
 19. Thesystem according to claim 17, wherein the processor is configured tocause the second transceiver to establish communication with theterminal using same communication parameters used for communicationbetween the first transceiver and the terminal.
 20. The system accordingto claim 17, wherein the processor is configured to cause the secondtransceiver to establish communication with the terminal withoutnegotiating communication parameters between the second transceiver andthe terminal.
 21. The system according to claim 17, wherein theprocessor is configured to transfer to the second transceiver one ormore communication parameters used for communication between the firsttransceiver and the terminal, and to cause the second transceiver tocommunicate with the terminal using the communication parameters. 22.The system according to claim 17, wherein, after switching to the secondchannel, the processor is configured to determine that the secondtransceiver is unsuitable for communicating with the terminal, and totake an alternative action.
 23. The system according to claim 22,wherein the processor is configured to take the alternative action byperforming one of: instructing the terminal to switch back to the firsttransceiver; instructing the terminal to switch to a third transceiver;initiating communication with the terminal over an alternatecommunication protocol; and denying communication with the terminal. 24.The system according to claim 17, wherein the processor is configured toestimate a quality metric for the first channel, and to decide whetherto roam to the second channel based on the estimated quality metric. 25.The system according to claim 17, wherein, by sending the channel switchcommand, the processor is configured to balance a traffic load betweenthe first and second transceivers.
 26. The system according to claim 17,wherein the processor is configured to assess a traffic need of theterminal, and to decide whether to roam to the second channel based onthe assessed traffic need.
 27. The system according to claim 17, whereinthe processor is configured to assess a traffic capability of the firstor second transceiver, and to decide whether to roam to the secondchannel based on the assessed traffic capability.
 28. The systemaccording to claim 17, wherein, before sending the channel switchcommand, the processor is configured to carry out a channel selectionprocess that assigns multiple channels to the multiple respectivetransceivers, including assigning the first and second channels to thefirst and second transceivers, respectively.
 29. The system according toclaim 28, wherein the processor is configured to assign a given channelto at least two of the multiple transceivers.
 30. The system accordingto claim 29, wherein the processor is configured to coordinate access tothe given channel among the two or more transceivers assigned to thegiven channel.
 31. The system according to claim 28, wherein theprocessor is configured to disable one or more of the multipletransceivers in order to assign each transceiver that is not disabled adifferent respective channel.
 32. The system according to claim 28,wherein the processor is configured to distribute multiple terminals,including the wireless communication terminal, among the multipletransceivers by evaluating the roaming condition and instructing thewireless communication terminal to switch.